Microstructural Analysis of Functionally Graded Al–SiC–MWCNT Metal Matrix Composites by microscopy techniques

Functionally graded metal matrix composites are an attractive strategy to locally tailor stiffness, wear resistance, and thermal behavior in lightweight structural components by varying reinforcement content through the thickness of a single part. In aluminum (Al)-based systems, combining ceramic particles such as silicon carbide (SiC) with multi-walled carbon nanotubes (MWCNTs) enables simultaneous control of load transfer, crack deflection, and interfacial architecture, while maintaining low density and good processability [1-3]. This work focuses on the microstructure characterization by microscopy techniques of hybrid Al–SiC–MWCNT functionally graded composites fabricated by a sequential powder metallurgy route. Commercially pure Al powder, SiC particles, and carboxyl-functionalized MWCNTs were used as raw materials; each constituent exhibits distinct morphology that governs blending and packing during processing. Al powder image evidence flake morphologies that favor dense packing, SiC particles present angular morphologies that promote mechanical interlocking, and MWCNTs with high-aspect-ratio(Fig. 1). To obtain a through-thickness gradient, powder batches were formulated, each containing a set of five compositions with systematically varied Al, SiC, and MWCNTs contents, which were homogenized by laboratory ball milling to promote dispersion of both reinforcements within each composition. Every composition in a set was uniaxially pressed into an individual green plate, and the five plates were then stacked in a prescribed order from an Al-rich layer to a highly reinforced layer to build the graded preform before hot compaction. The assembled stack was finally densified by hot pressing at 600 °C under a uniaxial pressure of 200 bar, with a holding stage at temperature that ensured metallurgical bonding across the five interfaces and stabilized the functionally graded architecture through the thickness. The resulting functionally graded compacts incorporate a smooth, layer-wise variation of SiC and MWCNTs contents. Scanning electron microscopy (SEM) of cross-sections at representative positions through the thickness reveals that, at low SiC content, the microstructure is dominated by a continuous Al matrix with sparse, well-separated SiC particles and a fine dispersion of MWCNT-rich features (Fig. 2a). In an intermediate layer, the content of SiC particles increases and the particles become more uniformly distributed while the MWCNTs dispersion remains similar, producing a more refined hybrid microstructure (Fig. 2b). In the high concentration of SiC layer, the matrix contains a dense particulate network, with MWCNTs agglomerates evidenced by an elongated morphology (Fig. 2c). A cross-sectional view across an interlayer transition zone between adjacent graded layers shows a continuous Al matrix that bridges both compositions and a gradual change in the local density of SiC particles and MWCNTs rich regions, confirming that the stacking and hot compaction promote a smooth microstructural transition through the composite (Fig. 2d). Energy-dispersive X-ray spectroscopy (EDS) mapping corroborates the designed reinforcement distribution by tracking the elemental signals of Al, Si, and C in layers with different SiC-MWCNT contents (Fig. 3). Two representative compositions with low and high SiC contents are shown side by side, enabling direct comparison of their elemental distributions. In both cases, the Al signal is dominant and spatially continuous, confirming the role of Al as a continuous matrix. The Si signal clearly marks the SiC particles, with a higher areal density of Si-rich regions in the 15 wt.% SiC layers than in the 5 wt.% SiC layers. The C maps highlight carbon-rich zones associated with MWCNTs, revealing their presence around SiC particles and within the matrix, and evidencing the change in C distribution when the SiC and MWCNTs contents are interchanged between the two layers. Qualitatively, the low porosity evidenced in the images, reflects the interplay between reinforcement content, packing efficiency, and densification behavior during hot compaction. SEM observations show that layers with low to intermediate reinforcement levels are largely dense, with only isolated, small pores mostly located at triple grain junctions or at the periphery of SiC particles where local rearrangement during sintering is less effective. In the highest-reinforcement layers, same behavior is evidenced. Across the whole graded section, there is no evidence of continuous porosity networks; instead, pores appear as discrete, rounded or irregular cavities embedded in an otherwise well-sintered matrix, suggesting that the selected powder metallurgy route is suitable to achieve substantial densification even in hybrid, highly reinforced layers (Fig. 3). Finally, SEM and EDS results demonstrate that the designed Al–SiC–MWCNTs functionally graded architecture is effectively transferred from the powder stage to the final consolidated compact, with a clear, controllable evolution of reinforcement content and morphology across the thickness. Layers with higher SiC and MWCNT contents exhibit a qualitatively higher densification. Although some microstructural inhomogeneities and localized porosity remain in the most heavily reinforced regions, the overall architecture suggests a favorable balance between reinforcement dispersion and matrix continuity that is indicative of improved mechanical performance compared to monolithic aluminum or non-graded counterparts with similar average composition. These observations underscore that sequential layer-wise powder metallurgy is an adequate and flexible route to engineer microstructure and reinforcement gradation in hybrid Al–SiC–MWCNT systems, enabling controlled densification and microstructural refinement tailored to microscopy-guided design objectives. Scanning electron microscopy (SEM) images of the raw materials: a) aluminum matrix powder, b) Multi-walled carbon nanotubes (MWCNTs), and c) silicon carbide (SiC) particles. SEM cross-sections images of the functionally graded Al–SiC–MWCNT composite: a) low content of SiC particle, b) intermediate content of SiC particle, c) high content of SiC particle, and d) interlayer transition zone between adjacent graded layers. SEM–EDS maps of graded Al–SiC–MWCNT layers: a) 5 wt.% SiC, 1.5 wt.% MWCNTs, and b) 15 wt.% SiC, 0.5 wt.% MWCNTs, showing the corresponding Al, C and Si distributions.